To your first post: There are a lot of answers possible...
You need to provide a real example and your metadata search setup. By the way, what plugin do you mean?
You can take a look in your log file of your metadata request. You will find it under "view log" in the download metadata window.

To your second post: Books have only one front cover. If you have 2 or more original cover then the books are different. This happens when the book is published with different book-idís. You need then make a decision about what the one you like is.